Embodiments of the invention relate generally to x-ray tubes and, more particularly, to an anti-fretting coating for a rotor attachment joint and a method of making same.
Computed tomography x-ray imaging systems typically include an x-ray tube, a detector, and a gantry assembly to support the x-ray tube and the detector. In operation, an imaging table, on which an object is positioned, is located between the x-ray tube and the detector. The x-ray tube typically emits radiation, such as x-rays, toward the object. The radiation typically passes through the object on the imaging table and impinges on the detector. As radiation passes through the object, internal structures of the object cause spatial variances in the radiation received at the detector. The detector converts the received radiation to electrical signals and then transmits data received, and the system translates the radiation variances into an image, which may be used to evaluate the internal structure of the object. One skilled in the art will recognize that the object may include, but is not limited to, a patient in a medical imaging procedure and an inanimate object as in, for instance, a package in an x-ray scanner or computed tomography (CT) package scanner.
A typical x-ray tube includes a cathode that provides a focused high energy electron beam that is accelerated across a cathode-to-anode vacuum gap and produces x-rays upon impact with an active material or target provided. Because of the high temperatures generated when the electron beam strikes the target, typically the target assembly is rotated at high rotational speed for purposes of spreading the heat flux over a larger extended area. The target is attached to a support shaft, which is in turn supported by roller bearings that are typically hard mounted to a base plate.
As such, the x-ray tube also includes a rotating system that rotates the target for the purpose of distributing the heat generated at a focal spot on the target. The rotating subsystem is typically rotated by an induction motor having a cylindrical rotor built into an axle that supports a disc-shaped target and an iron stator structure with copper windings that surrounds an elongated neck of the x-ray tube. The rotor of the rotating subsystem assembly is driven by the stator.
During manufacturing, the rotor may be attached to the axle of the rotating subsystem using for instance a weld or a bolted joint. In the case of a welded attachment an adequate joint for joining the rotor to the axle can typically be formed using common and known welding techniques. However, welded joints can be costly, both in terms of the manufacturing process but also in terms of inspection and rework costs. The costs of a weld joint for the rotor are also compounded because often the welding is performed in a clean environment, necessitating special care to maintain cleanliness and to reduce particulate emission.
In the case of a bolted joint, fabrication and assembly costs can be significantly reduced overall when compared to a welded joint. However, such bolted joints are subject to wear and early life failure for a number of reasons. First off, relative motion can occur between components, due at least in part to a mismatch of thermal expansion coefficients of the materials that are typically on either side of the bolted joint. As the parts heat up during x-ray tube operation, the thermal coefficient mismatch causes a mismatch in the amount of expansion of the components, enabling the components to slide relative to each other. This manifests itself in the form, typically, of radially oriented fretting that occurs at the face of the materials that make up the bolted joint.
Secondly, the cyclical nature of the joint loading can cause relative motion in the joints as well. Because the target is typically rotated about its axis at a high rate of speed, typically 100 Hz or more, and because the x-ray tube itself is rotated at a high rate of speed on a gantry, typically 2 Hz or more, enormous periodic or cyclical loads can be generated at interfaces that join the rotor to the bearing axle or shaft. So, high-frequency periodic loads are applied to the joint due to the target rotation and some unavoidable residual unbalance of the rotating components and low-frequency periodic loads due to the tube rotation on the CT gantry. Such loads can cause bending of the rotor joint components causing small relative circumferential motion to occur, which can cause circumferentially oriented fretting that occurs at the face of the materials that make up the bolted joint.
In order to reduce the amount of fretting that occurs in the bolted joint, parts may be pressfit together as well in order augment the pressure between components. Thus, an interference fit may be formed that couples or otherwise attaches the rotor to the bearing shaft, which are then bolted together as well. However, despite having an improved joint, fretting and particulate generation can nevertheless occur therein. In fact, particles can be generated at any interface where materials are in a bolted joint or in an interference fit pressed together. And, the effect can increase significantly with increased gantry and/or increased target rotating speed, leading to increased fretting and particulate generation as x-ray tubes are rotated faster on gantries and as targets are rotated faster within x-ray tubes.
As known in the art, particulate in an x-ray tube can degrade performance and life in a number of ways that include, for instance, accelerated bearing wear if the wear particles fall into the bearing and electrical discharge activity in the high voltage environment of the x-ray tube. Both of these issues reduce the useful life of the x-ray tube.
Accordingly, it would be advantageous to have an x-ray tube that could be rotated at a high speed on a gantry and at a high target rotational speed without a reduction in life due to particulate generation at connection joints in the x-ray tube.